1. Field of the Invention
The present invention is in the field of capacitance diaphragm gauges which measure pressure based on the deflection of a diaphragm.
2. Description of the Related Art
Absolute capacitance diaphragm gauges (CDGs) measure pressure by sensing the capacitance change associated with deflection of a diaphragm whereby one side of the diaphragm (“the Px side”) is exposed to the pressure to be measured (Px) and the other side of the diaphragm is exposed to a sealed reference vacuum cavity in which an ultrahigh vacuum (e.g., less than 10−9 Torr) has been created prior to the sealing of the reference cavity.
The CDG measures capacitance between a diaphragm and one or more fixed electrodes housed in the reference vacuum cavity. When the pressure on the Px side of the diaphragm is higher than the pressure in the reference vacuum cavity, the diaphragm deflects in the direction of the fixed electrode (or electrodes), which increases the measured capacitance. As the pressure on the Px side of the diaphragm decreases, the pressure differential across the diaphragm diminishes and the diaphragm moves away from the fixed electrode (or electrodes) in the reference vacuum cavity, which reduces the measured capacitance.
As the pressure on the Px side of the diaphragm approaches the pressure in the reference vacuum cavity, the pressure differential across the diaphragm becomes sufficiently small as to be considered as the “zero point” for the CDG. This fixed zero point is established during the calibration of the CDG and is used as a reference in subsequent pressure measurements.
CDGs are commonly used to the measure pressure in vacuum chambers in which thin or thick films of material are deposited on a substrate. One common example of usage is to measure pressure during the deposition of materials onto the surface of silicon wafers during the fabrication of semiconductor devices. CDGs are quite useful in vacuum deposition processes that utilize multiple gasses because capacitance diaphragm gauges are highly accurate and are able to measure absolute pressure independent of gas composition.
The accuracy of the measurement of pressure by a CDG can be negatively impacted by several factors, one of which is the integrity of the sealed reference vacuum cavity. As set forth above, the pressure within the reference vacuum cavity is quite low. The pressure must remain substantially constant in order to provide a constant reference pressure against which the pressure on the Px side of the diaphragm is measured. Any change in the pressure within the reference vacuum cavity will create a shift in the zero point of the CDG.
Although the reference vacuum cavity of the CDG is sealed, gas molecules may occur within the reference vacuum cavity and degrade the integrity of the reference vacuum cavity after the cavity is sealed. Common sources of gas molecules include, for example, outgassing from the internal surfaces of the reference cavity, leaks that may occur because of improper manufacturing techniques, diaphragm failure, and pinhole leaks in the diaphragm. When the rate that gas molecules enter the reference vacuum cavity and the resulting quantity of the molecules within the reference vacuum cavity are sufficiently low, the molecules that occur are substantially absorbed by getter material encapsulated in the reference vacuum cavity. Thus, the getter material is able to maintain the integrity of the reference vacuum cavity and prevent any shift in the zero point of the CDG.
Under some circumstances, the getter material is unable to sufficiently absorb all of the gas molecules entering the reference vacuum cavity, and the pressure in the reference vacuum cavity rises accordingly. In such circumstances, the CDG exhibits a negative zero shift which adversely impacts the accuracy of the CDG. The adverse effect on the accuracy of the CDG is often a cause of concern for users of CDGs. The problem is compounded because the reference vacuum cavity is a fully sealed assembly. Thus far, the sealed reference vacuum cavity has made it impossible for the user to detect the occurrence or the magnitude of any degradation of the reference vacuum cavity caused by gas molecules within the reference vacuum cavity. Furthermore, a CDG may experience other common phenomena that can cause the CDG to exhibit a negative zero shift. Such common phenomena have nothing to do with a degradation in the reference vacuum cavity; and, to date, there has been no way to distinguish between actual gas leaks into the reference vacuum cavity and other phenomena that create a similar negative zero shift symptom. This has made CDG diagnostics particularly problematic and has resulted in significant lost time and money for users of CDG devices.